Recently, in optical transmission devices, improvement in frequency use efficiency has been desired with increasing traffic demand. In view of this, in such an optical transmission device, a driving signal is electrically filtered to narrow the bandwidth of a modulated optical signal, whereby the frequency use efficiency can be improved.
In the optical transmission device, as an optical modulator of a quadrature phase shift keying (QPSK) modulation scheme mainly used in, for example, a 100-GHz coherent optical communication system, for example, a Mach-Zehnder modulator (MZM) is used. Furthermore, the MZM has an I (In-phase channel)-arm and a Q (Quadrature-phase channel)-arm and, in order to cause biases of the I-arm and the Q-arm by using a modulated optical signal at an output stage to converge to optimum points, uses automatic bias control (ABC) of controlling the respective bias values. Conventional examples are described in Japanese National Publication of International Patent Application No. 2014-516480, Japanese Laid-open Patent Publication No. 2012-217127, Japanese Laid-open Patent Publication No. 2013-127519, Japanese Laid-open Patent Publication No. 2013-126050, and Japanese Laid-open Patent Publication No. 2008-092172.
However, electrically filtering the driving signal causes the driving signal to become smaller, which increases the number of convergence points used as indices when the ABC is performed, thereby causing false convergence. Besides, although there are a plurality of convergence points, there is only one correct convergence point.
Furthermore, in the optical modulator, when the average driving amplitude of the driving signal is small, the monitor sensitivity decreases, which needs more time until the biases are optimized, thereby increasing processing load therefor. In other words, until the biases are optimized, a significantly long period of time is needed to perform the ABC again and change initial biases for restart.